1. Field of the Invention
This invention relates to the separation of normal paraffins from hydrocarbon mixtures thereof with non-normal hydrocarbons. More particularly, it relates to the separation of said normal paraffins from gas oil-containing feedstocks.
2. Description of the Prior Art
An isobaric process for the separation of normal paraffins from a hydrocarbon vapor feed stream having 10-25 carbon atoms per molecule and containing a mixture of said normal paraffins and non-normal hydrocarbons is disclosed in the Avery U.S. Pat. No. 3,422,005. The feedstock may contain gas oil having 16 to 25 carbon atoms, kerosene having 10 to 15 carbon atoms, or a mixture thereof. The patent discloses the steps of (1) adsorption, i.e. selective adsorption of normal paraffins, (2) cocurrent purge with n-hexane to sweep out void space vapor containing a high concentration of non-adsorbable components, i.e. non-normal hydrocarbons, from the upper of effluent end of the bed, (3) countercurrent purge with n-hexane to desorb normal hydrocarbon adsorbate from the bed, the highest molecular weight, adsorbed normal hydrocarbons i.e., paraffins, being concentrated near the bottom or feed end of the bed. The effluent from the upper end of the bed is cooled and passed to a non-normal dehexanizer column from which non-normal hydrocarbons are withdrawn as a liquid bottoms product. The effluent from the bottom end of the bed is cooled and passed to a normal paraffin dehexanizer column from which normal paraffin bottoms are withdrawn. The n-hexane discharged as overhead from said columns is transferred to storage as liquid and is subsequently heated and used as purge fluid as indicated above. The advantages of employing n-hexane as the purge fluid and of employing a relatively high isobaric adsorption-desorption pressure level, together with a relatively low adsorption-desorption temperature range, are set forth in the Avery patent.
It has been found that the separation process should be carried out a temperature above the dew point of the hydrocarbon feed and sufficiently high to avoid capillary condensation. Operating at such a temperature serves to avoid capillary condensation and the formation of a liquid meniscus in the macropores of pellets. If such a precaution were not taken, the isomer condensate in the adsorbent macropores would not be completely removed during the copurge or displacement step, and the normal paraffin purity as well as the separation of adsorbed and unadsorbed components would be lower than in an all-vapor process. It is possible to avoid capillary condensation by ensuring that the ratio of feed saturation pressure (i.e. dew point pressure) to operating pressure is more than about 2. For this purpose, a gas oil feedstock having, for example, a dew point of 670.degree. F. at a typical operating pressure of 25 p.s.i.a. should be contacted with a molecular sieve adsorbent at a temperature of about 730.degree. F. At such a temperature, however, excessive cracking of the gas oil vapor feed occurs, with coke formation and rapid deactivation of the coke resulting therefrom. For this reason, it is preferred to operate at temperatures of between about 600.degree. and 700.degree. F. with gas oil feedstocks. In this regard, it should be noted that the cracking and deactivation rates increase with increasing molecular weights, and the problems are less serious with respect to the lighter kerosene feedstocks. A particular problem exists, therefore, in the processing of gas oil feedstocks so as to operate at a temperature sufficiently high to avoid capillary condensation without encountering significant cracking and deactivation problems.
Avery discloses the overcoming of this problem by the introduction of sufficient redistilled n-hexane purge into the gas oil-containing feed to lower the resulting mixture's dew point and to avoid capillary condensation at the desired operating pressure so as to permit operation at a temperature below 700.degree. F. As the adsorbent already contains normal hexane from the previous purge step, the n-hexane introduced into the feed for dilution thereof is discharged from the bed with the unadsorbed non-normal hydrocarbons and the previously adsorbed n-hexane. This effluent is fractionated as indicated above, with the n-hexane overhead fraction being recycled for use as purge gas or for mixing with the feed material.
It has also been proposed to recycle the adsorption effluent, i.e. non-normal hydrocarbon product, to the feedstock for the desired dilution thereof. While such techniques are useful in overcoming capillary condensation while enabling temperature below 700.degree. F. to be employed, each is accompanied by disadvantages that limit the overall economy and effectiveness of the separation process. Thus, the use of redistilled n-hexane for feedstock dilution increases the size of the equipment employed and the amount of energy consumed in the processing of the n-hexane recycle stream, thereby increasing the cost of the overall operation. The use of the adsorption effluent for such dilution purposes tends to increase the amount of non-normals in the feed and also the amount of normal paraffins present in the product effluent, reducing the normals as well as the non-normal hydrocarbon product purity and the separate recovery of n-paraffin material. There is a need in the art, therefore, for improvements in the process for separating normal paraffins from hydrocarbon mixtures, particularly as exist in gas oil feedstocks.
It is an object of the invention to provide an improved process for the separation of normal paraffins from hydrocarbon feedstocks.
It is another object of the invention to provide an improved process for the separation of normal paraffins from gas oil feedstocks without capillary condensation effects.
It is a further object of the invention to provide a process for the effective separation of normal paraffins from gas oil feedstocks at temperatures below 700.degree. F.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.